1. Field of the Invention
The present invention relates in general to tone event detection and enhancement, and more specifically to an enhanced tone detector with and adaptive multi-bandpass filter which improves tone detection and enhances performance during high noise signaling conditions.
2. Description of the Related Art
Tone event detection is an important application in digital telephony systems having shared data and voice communication over the telephone network. The International Telecommunication Union (ITU) has been developing a standard for data transmission (V-series modem signals) over packet networks (V.150), which defines inter-operation between Public Switched Telephone Network (PSTN) and Internet Protocol (IP) networks. Prior to or during data transmission, a series of tone events are exchanged, so that whenever a tone event is detected, specific actions need to be taken for initiating and maintaining proper data communication over IP.
A tone event may include a single tone or a combination of multiple tone segments having different modulation schemes. For example, a tone event may include a single frequency tone with an on/off amplitude modulation (AM). In another example, a tone event may include a dual frequency component tone followed by a single frequency tone. In yet another example, a tone event may include a single frequency tone having periodic phase changes. Therefore, it is desirable to reliably detect tone events according to different modulation schemes. Some typical tone events that may be detected include T.30 Calling Tone (CNG), V.25 Calling Tone (CT), V.25 Answering Tone (ANS)/T.30 CED, V.25 Answering Tone with Phase Reversal (ANS_PR), V.8 Answering Tone with Amplitude Modulation (ANSam), V.8 Answering Tone with Amplitude Modulation and Phase Reversal (ANSam_PR), V.22 Unscrambled Binary Ones (USB1), V.21 Channel 2 HDLC flag, TIA/EIA-825 TTY tones, V.8bis Initiate/Respond, Q.24 Dual Tone Multiple Frequency (DTMF), Multiple Frequency Tones (e.g. MFR1, MFR2), etc. These tone events are modulated by magnitude, frequency, or phase, or any combination thereof. Furthermore, these tone events may include more than one frequency component per segment of the signal having the same tone characteristics.
Tone detectors are frequently used in communication systems for monitoring the receive signals and identifying the presence of pre-defined signaling tones. There are many practical situations, however, when signaling tones are highly corrupted by noise, which may end up disrupting expected tone detection functionality. New tone detection methods have been developed which attempt to handle noisy situations via post-processing low-pass filtering. Due to frequency and timing tolerance requirements, however, missed detection rates tend to increase as noise distortion increases beyond critical levels. Therefore, there is a need to identify noisy conditions, and selectively filter the signal to increase signal-to-noise ratio (SNR) in order to expand tone detection operation range.